The glaucomas are a heterogeneous group of optic neuropathies characterized by cupping of the optic nerve head, thinning of the retinal nerve fiber layer due to loss of retinal ganglion cells, and specific pathognomonic changes in visual fields. Elevated intraocular pressure (IOP) is a very important risk factor for the development of most common forms of glaucoma (Sommer A, et al. Relationship between intraocular pressure and primary open angle glaucoma among white and black Americans. Arch. Ophthalmol. 109:1090-1095,1991).
A family history of glaucoma also is an important risk factor for the development of glaucoma. It appears that a significant portion of glaucoma is inherited (or at least the risk for developing glaucoma is inherited) although it is often difficult to establish clear inheritance patterns for most of the glaucomas because of the disease onset late in life and the slowly progressive clinical manifestations of the disease. Despite these problems, a number of families with heritable forms of glaucoma have been identified and these families have been used to map a variety of glaucoma genes (Sheffield, et al., "Genetic Linkage of Familial Open Angle Glaucoma to Chromosome 1q21 -q31," Nature Genetics, 4:47-50 (1993); Sarfarazi, et al., "Assignment of a Locus (GLC3A) for Primary Congenital Glaucoma (Buphthalmos) to 2p21 and Evidence for Genetic Heterogeneity," Genomics, 30:171-177 (1995); Akarsu, et al., "A Second Locus (GLC3B) for Primary Congenital Glaucoma (Buphthalmos) Maps to the 1p36 Region," Human Molecular Genetics, 5(8):1199-1203 (1996); Stoilova, et al., "Localization of a Locus (GLC1B) for Adult-Onset Primary Open Angle Glaucoma to the 2cen-q13 Region," Genomics, 36:142-150 (1996); Wirtz, et al., "Mapping a Gene for Adult-Onset Primary Open-Angle Glaucoma to Chromosome 3q," Am. J Hum. Genet., 60:296-304 (1997); Andersen, et al., "A Gene Responsible for the Pigment Dispersion Syndrome Maps to Chromosome 7q35-q36," Arch. Ophthalmol., 115:384-388 (1997)). The first glaucoma gene mapped (GLC1A) was in a large family with autosomal dominant inherited juvenile glaucoma (JG). This disease is characterized by an early disease onset (late teens to early 20s), relatively high IOPs, and general resistance to conventional pharmacological IOP lowering therapy. The GLC1A gene was mapped by positional cloning and linkage analysis to chromosome 1q22-q25 (Sheffield et al, Id.) and a number of other groups have confirmed the 1q location of this juvenile glaucoma gene (Richards, et al., "Mapping of a Gene for Autosomal Dominant Juvenile-Onset Open-Angle Glaucoma to Chromosome 1q," Am. J. Hum. Genet., 54:62-70 (1994); Morissette, et al., "A Common Gene for Juvenile and Adult-Onset Primary Open-Angle Glaucomas Confined on Chromosome 1q," Am. J. Hum. Genet., 56:1431-1442 (1995); Wiggs, et al., "Genetic Linkage of Autosomal Dominant Juvenile Glaucoma to 1q21-q31 in Three Affected Pedigrees," Genomics, 21:299-303 (1994); Meyer, et al., "Age-Dependent Penetrance and Mapping of the Locus for Juvenile and Early-Onset Open-Angle Glaucoma on Chromosome 1q (GLC1A) in a French Family," Hum. Genet., 98:567-571 (1996); Graff, et al., "Confirmation of Linkage to 1q21-31 in a Danish Autosomal Dominant Juvenile-Onset Glaucoma Family and Evidence of Genetic Heterogeneity," Hum. Genet., 96:285-289 (1995)). Glaucoma due to the GLC1A gene is often referred to as 1q glaucoma.
The GLC1A gene was identified as encoding a 57 kD protein expressed in the trabecular meshwork (TM) (Stone, et al., "Identification of a Gene That Causes Primary Open Angle Glaucoma," Science, 275:668-670 (1997)). The expression of the GLC1A gene, and the encoded TM protein, is up-regulated by glucocorticoids (Polansky, et al., "Eicosanoid Production and Glucocorticoid Regulatory Mechanisms in Cultured Human Trabecular Meshwork Cells," The Ocular Effects of Prostaglandins and Other Eicosanoids, pp. 113-138 (1989); Polansky, et al., "In Vitro Correlates of Glucocorticoid Effects on Intraocular Pressure," Glaucoma Update IV (1991); and Polansky, et al., "Cellular Pharmacology and Molecular Biology of the Trabecular Meshwork Inducible Glucocorticoid Response Gene Product," Ophthalmologica, 211:126-139 (1997)). This TM protein is also known as TIGR (trabecular meshwork inducible glucocorticoid response) (Polansky, Id.). The glucocorticoid-induction of this TM protein has been suggested to be involved in the generation of glucocorticoid-induced ocular hypertension and glaucoma (Polansky, Id.).
The GLC1 A gene is expressed in other ocular tissues such as the ciliary epithelium (Ortego, et al., "Cloning and Characterization of Subtracted cDNAs from a Human Ciliary Body Library Encoding TIGR, a Protein Involved in Juvenile Open Angle Glaucoma with Homology to Myosin and Olfactomedin," FEBS Letters, 413:349-353 (1997)) and the retina (Kubota, et al., "A Novel Myosin-like Protein (Myocilin) Expressed in the Connecting Cilium of the Photoreceptor: Molecular Cloning, Tissue Expression, and Chromosomal Mapping," Genomics, 41:360-369 (1997)). The gene is referred to by several names including GLCIA (Sheffield, supra; Sunden, et al., "Fine Mapping of the Autosomal Dominant Juvenile Open Angle Glaucoma (GLC1A) Region and Evaluation of Candidate Genes," Genome Research, 6:862-869 (1996); Stone, et al., supra), TIGR (Polansky supra; Ortego, supra), and myocilin (Kubota, supra). Mutations GLC1A are not only responsible for juvenile glaucoma, but also a significant subset of adult onset primary open angle glaucoma (Stone, et al., supra; Adam, et al., "Recurrent Mutations in a Single Exon Encoding the Evolutionarily Conserved Olfactomedin-Homology Domain of TIGR in Familial Open-Angle Glaucoma," Human Molecular Genetics, 6(12):2091-2097 (1997)). The 1q glaucoma gene (GLC1A, TIGR) is the subject of Nguyen, et al., U.S. Pat. No. 5,606,043, issued Feb. 25, 1997.
Glucocorticoids have been associated with the development of ocular hypertension and primary open angle glaucoma (Kass, et al., "Corticosteroid-Induced Glaucoma, In Ritch, R., Shields, M. B., Krupin, T. (eds.)," The Glaucomas, The C. V. Mosby Company, St. Louis, Miss., pp. 1161-1168 (1989); DeSantis, et al., "Dexamethasone-Induction of Ocular Hypertension in the Primate, ARVO Abstracts. Invest. Ophthalmol. Vis. Sci., 31(Suppl.):99 (1990); Knepper, et al., "Intraocular Pressure and Glycosaminoglycan Distribution in the Rabbit Eye: Effect of Age and Dexamethasone," Exp. Eye Res., 27:567-575 (1978); Francois, et al., "Ultrastructural and Morphometric Study of Corticosteroid Glaucoma in Rabbits, Ophthalmic Res., 16:168-178 (1984); Lorenzetti, O. J., "Effects of Corticosteroids on Ocular Dynamics in Rabbits," J. Pharmacol. Exp. Therap., 175:763-772 (1970); and Zhan, et al., "Steroid Glaucoma: Corticosteroid-Induced Ocular Hypertension in Cats," Exp. Eye Res., 54:211-218 (1992)). Glaucoma patients have also been reported to have higher levels of the endogenous glucocorticoid, cortisol (Rozsival, et al., "Aqueous Humour and Plasma Cortisol Levels in Glaucoma and Cataract Patients," Current Eye Research, 1:391-396 (1981); Ray, et al., "Plasma Cortisol in Glaucoma," Ann. Ophthalmol., 9:1151-1154 (1977); and Schwartz, et al., "Increased Plasma Free Cortisol in Ocular Hypertension and Open Angle Glaucoma," Arch. Ophthalmol., 105:1060-1065 (1987)).
It is known that trabecular meshwork cells have glucocorticoid receptors and that glucocorticoid binding with these receptors causes a change in trabecular meshwork cell gene expression. Known manifestations of this change include a reorganization of the cytoskeleton (Wilson, et al., "Dexamethasone Induced Ultrastructural Changes in Cultured Human Trabecular Meshwork Cells, Cur. Eye Res., 12:783-793 (1993), and Clark, et al., "Glucocorticoid-Induced Formation of Cross-Linked Actin Networks in Cultured Human Trabecular Meshwork Cells," Invest. Ophthalmol. Vis. Sci., 35:281-294 (1994)) and increased deposition of the extracellular matrix material in trabecular meshwork cells. As a result, the trabecular meshwork becomes "clogged" and unable to perform one of its most critical functions, that is, serving as a gateway for aqueous humor flow from the anterior chamber of the eye. When the aqueous humor flow out of the eye via the trabecular meshwork is diminished, the intraocular pressure of the eye rises. If this state of elevated intraocular pressure is maintained or frequently occurs, the optic nerve head can be damaged resulting in the loss of visual field. Loss of visual field is the hallmark symptom associated with glaucoma.
Endogenous glucocorticoids may be responsible for producing the changes in the trabecular meshwork that lead to ocular hypertension and glaucoma. In the present case, it is believed that non-steroidal glucocorticoid antagonists bind to the glucocorticoid receptor in trabecular meshwork cells, and thereby prevent binding of endogenous glucocorticoids to the glucocorticoid receptor. They may also displace endogenous glucocorticoids which are bound to glucocorticoid receptors.
In summary, the GLC1A gene product can lead to the development of ocular hypertension and glaucoma in one of two ways: (1) mutations in GLC1A are responsible for most forms of juvenile glaucoma and a subset of adult onset POAG or (2) exposure of some individuals to glucocorticoids leads to increased GLC1A expression in the TM which causes increased aqueous humor outflow resistance and the development of ocular hypertension. The precise mechanism(s) responsible for GLC1A effects on IOP are currently unknown.